The present invention relates to turbulators used in compact tube heat exchangers for use in automotive applications.
It has been known to use thin metal sheet or foil which has been formed into corrugations in heat exchangers and to form such material with louvers to improve the heat exchange characteristics of the material. It has also been known to form corrugated material with alternate staggered portions so that the free edges of the portions are presented to the flow of fluid over the material when used in heat exchangers. An example of such material is disclosed in U.S. Pat. No. Re. 35,890 issued to So.
The thin metal sheets that are intended to generate artificial turbulence are generally referred to as turbulators or turbulizers and typically consist of sinusoidal convolutions or rectangular corrugations extending in rows axially along the length of a heat exchanger. Adjacent rows in the flow or axial direction are displaced from one another thereby creating transverse rows of transversely aligned parallel slits or apertures. The function of this geometry is to create artificial turbulence since as the hot fluid flows through the heat exchanger and impinges against the leading edge of the corrugations, the resulting excessive form drag splits the fluid flow sideways as it advances to the next row of corrugations. This artificial turbulence is desirable in that it results in enhanced heat transfer characteristics.
Current design trends in the automotive industry are towards more compact and aerodynamically efficient designs in an effort to increase fuel efficiency and accommodate new accessories such as pollution control devices and the like. These trends have led to a need to reduce the size of the radiator tank, and therefore more compact oil coolers are required. Accordingly, there is a need for smaller turbulators having widths substantially smaller than their lengths.
It has been known to produce corrugated material from sheets of raw material by rolling the material through a pair of cooperating rollers forming a nip and having surface enhancements and knives for forming the corrugations and for making the slits. An example of a roller system for producing corrugated sheet material is disclosed in U.S. Pat. No. 4,170,122 issued to Crowell. Some of the drawbacks to the rolling process include the cost of the rolls due to the surface enhancements for rolling the corrugations and the required width of the rolls. In rolling techniques the material is typically fed in a direction perpendicular to the longitudinal axes of the corrugations thereby requiring a wide roll for longer parts. The wide rollers require expensive tooling and larger machines. Also, once the corrugations are formed they have to be cut into strips at the desired width, and the cutting of the individual pieces has to be coordinated with the motion of the rollers. As a result, the accuracy of the rolls with regard to the height of the corrugations is somewhat limited.
As an alternative to rolling, a stamping process is desirable in that it reduces the cost of the machine, enables the part to be formed in the longitudinal direction corresponding to the longitudinal axes of the corrugations, and provides greater accuracy with regard to the shape of the corrugations and particularly the height. One of the problems with stamping thin sheets of aluminum is that the material is relatively brittle and the stamping process can result in failures such as cracking that may present themselves during the formation of the corrugations or during the slitting of the turbulator. It has been determined that in forming a multi-corrugated turbulator, the first corrugation is the most critical, and if the process of forming the first corrugation creates too much stress, the part will fail. The typical method for forming the initial corrugation is pressing the flat sheet of raw material in a die set between a solid punch and a die. The punch is a relatively sharp tool that even when rounded at the end may cause too much stress that results in cracking down the middle of the raw material in the axial direction.
Accordingly, what is needed is a device and method for forming relatively small, narrow turbulators in a stamping process without cracking and/or other stress related failures.
The present invention meets the above described need by providing a device and method for manufacturing a turbulator.
The present invention provides for manufacturing compact turbulators having lengths substantially larger than their widths and that are typically made of thin gauge metals.
The device provides a progressive die for use in a high-speed press for forming a turbulator having multiple rows of axial corrugations. The corrugations are slit and offset such that artificial turbulence is generated as the fluid passes through the corrugations. The device includes a plurality of progressive dies disposed along an axial material direction.
A flat strip of material enters the dies and is folded about its longitudinal axis in a relatively wide V-fold. As the strip of material moves forward, it is intermittently stamped in the series of dies. The initial dies create a central V-shaped fold that gradually narrows into a U-shaped channel with approximately straight walls.
Once the first corrugation is formed, a series of progressive dies form the remaining corrugations in alternating fashion. Next, the material moves through a slitting station that provides the turbulator with apertures and an axial offset in the axial direction.